Method of depositing phosphor dot triads on a cathode ray tube screen

ABSTRACT

THE FACEPLATE PANEL AND SHADOW MASK ASSEMBLY OF A COLOR CATHODE RAY TUVE ARE MOUNTED ON A LIGHTHOUSE IN A SPACED RELATION TO A POINT SOURCE OF LIGHT. A FORCE IS APPLIED TO THE SHADOW MASK SCREEN TO TILT THE SAME ABOUT THE SPRINGS CONNECTING IT TO STUDS IN THE FACEPLATE PANEL. WITH THE MASK IN THE TILTED POSITION, THE SPACING BETWEEN THE SHADOW MASK AND THE SCREEN WHEN DEPOSITING THE PHOSPHOR DOT TRIAD THEREON IS EQUAL TO THE SPACING BETWEEN THE SHADOW MASK AND THE SCREEN IN AN OPERATING INSTALLATION.

Apnl 6, 1971 R. e. O'FALLON 3,573,

METHOD OF DEPOSITING PHOSPHOR DOT TRIADS ON A CATHODE RAY TUBE SCREEN Filed May 10, 1968 '3 Sheets-Sheet 1 l8 U.L.

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Apnl 6, 1971 R. G. O'FALLON 3,573,909

METHOD OF DEPOSITING PHOSPHOR DOT TRIADS 1 I ON A CATHODE RAY TUBE SCREEN Filed May L0, 1968 3 Sheets-Sheet 2 24 0 O D 'O G O Q O 25A] 0 C) 24W O O O O O Q 2:25.

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f FIGG April 6, 1971 a, O'FALLQN 3,573,909

METHOD OF DEPOSITING PHOSPHOR now TRIADS ON A CATHODE RAY TUBE SCREEN Filed May 10, 1968 ,JSheets-Sheet 5 LIGHT SOURCE INVENTOR RICHARD s. O'FALLON BY '3 ?M ATTORNEYS.

United States Patent 3,573,909 METHOD OF DEPOSITING PHOSPHOR DOT TRIADS ON A CATHODE RAY TUBE SCREEN Richard G. OFallon, Westchester, Ill., assignor to Motorola, Inc., Franklin Park, Ill. Filed May 10, 1968, Ser. No. 728,299 Int. Cl. G030 5/00 US. Cl. 9636.1 6 Claims ABSTRACT OF THE DISCLOSURE The faceplate panel and shadow mask assembly of a color cathode ray tube are mounted on a lighthouse in a spaced relation to a point source of light. A force is applied to the shadow mask screen to tilt the same about the springs connecting it to studs in the faceplate panel. With the mask in the tilted position, the spacing between the shadow mask and the screen when depositing the phosphor dot triad thereon is equal to the spacing between the shadow mask and the screen in an operating installation.

BACKGROUND OF THE INVENTION Generally, today a photographic technique is employed to produce the screens for color cathode ray tubes. The technique includes covering the front panel with a slurry comprising phosphor, polyvinyl alcohol and ammonium dichromate sensitizer in water and drying the same. A shadow mask assembly is then inserted into the panel and the combination is mounted on a lighthouse where the screen is exposed to a point source of light located in a position relative to the apparent source of the beam from the particular electron gun associated with each color to be deposited on the screen. The light passes through apertures in the shadow mask and strikes the screen exposing the polyvinyl alcohol with the phosphor embedded in it. The shadow mask is removed, and the face panel is developed by washing with water to remove the polyvinyl alcohol and phosphor which are not exposed to the light. The process is done three times to complete a screen, with a different phosphor being used each time. When the screen is completed, it comprises a plurality of triads of red, blue and green phosphor dots which are tangentially dispersed over the entire screen surface.

Theoretically, when the tube is completed, each of the three electron guns which scan across the shadow mask screen will illuminate only those dots associated with that particular gun. in practice, however, there are certain errors between beam landings and phosphor dot locations. These errors are brought about, for instance, by the effect of stray magnetic fields on the travel of the electron beam from the gun to the screen. Another error is introduced because the electron guns are mounted in the neck of the tube in an equilateral triangle configuration so that one gun is positioned above the other guns. This introduces beam landing errors which are greatly magnified with high scan angles, i.e., when the beam is scanned to the edge of the tube.

It has been proposed to correct beam landing errors by positioning the point source of light in the lighthouse such that the screen will be exposed through a hole in the shadow mask other than the hole that the electron beam passes through when exciting the dot during tube operation. This method of correcting beam landing errors is not very desirable from several viewpoints. For instance, when exposing a cathode ray tube screen in this manner, the dots of the triad will not be tangential. Because of this, it is necessary to reduce the dot diameters so that they will not overlap each other. By reducing the dot diameter, the guard band (the difference be- 3,573,909 Patented Apr. 6, 1971 'ice tween the diameter of the dot and the diameter of the electron spot) is reduced. A reduction in guard band results in magnification of any beam landing error. Furthermore, if the beam misses the dot there is a good prob- 5 ability it will land on the screen in a blank space hetween adjoining dots such that there is no illumination whatsoever. This greatly degrades the black and White color purity. From a production standpoint, it is diflicult to train people to inspect the completed screen. For instance, it is easy to tell an inspector to be sure that all the dots are tangent; however, confusion reigns when the inspector is told that the dots are to be tangent in the middle of the screen and do not have to be tangent at the corners and sides. The end result is that tubes with faulty screens are not discovered until a much later stage in the television receiver production, when it is extremely costly to replace the screen.

Some color cathode ray tubes employ an internal beam shield attached to the shadow mask for, among other things, reducing X-ray emissions, and use a three point suspension system for mounting the shadow mask assembly to the faceplate panel. Frequently the springs used in the suspension system are connected to bimetallic strips, welded to the shadow mask, which provides temperature compensation for the shadow mask during tube warm-up. When this type of tube is exposed in the lighthouse the shadow mask and panel are not in the vertical plane. When the tube is assembled into a television receiver, however, the faceplate panel and shadow mask assembly are in a vertical position. The weight of the shield on the shadow mask assembly establishes a moment of force which causes a slight tilting of the shadow mask about the three mounting studs on the faceplate panel. This tilting shifts the apertures of the shadow mask screen on the order of l to 2 mils. This slight motion, however, is sufiicient to introduce beam landing errors. In other tubes without internal shields, the weight of the shadow mask assembly itself can be sutficient to cause the tilting of the mask assembly about the stubs.

SUMMARY It is an object of this invention to provide an improved process for depositing phosphor dot triads on the screen of the faceplate panel of a color cathode ray tube.

It is another object of this invention to provide an improved process for depositing phosphor dot triads on the screen of a faceplate panel of a color cathode ray tube which are tangent to one another.

It is a further object of this invention to provide a process for depositing phosphor dot triads on the screen of the faceplate panel of a color cathode ray tube that permits simple and rapid inspection for defects thereby increasing reliability and decreasing production time.

It is still another object of this invention to provide a process for depositing phosphor dot triads on the screen of a color cathode ray tube of the type having an internal shield and using a three point suspension of the shadow mask screen wherein beam leading error introduced by the rotation of the screen about the suspension points due to the weight of the internal shield is reduced.

In one embodiment of this invention, a color cathode ray tube of the shadow mask type uses three electron guns for projecting electron beams through the apertures in the shadow mask assembly to illuminate phosphor dot triads on the screen of the front panel. A unique process is used to deposit the pattern of phosphor dot triads on the screen of the front panel of the cathode ray tube to reduce beam landing error. A photosensitive slurry is deposited on the front panel. The shadow mask assembly is then mounted to the front panel in a three point suspension, and the combination is positioned in a lighthouse in a spaced relation to a point source of light. A pivotally mounted arm having a weight on one end is actuated and engages the shadow mask assembly to tilt the same relative to the front panel so that the spacing of the shadow mask from the screen in the lighthouse, is equal to the spacing of the shadow mask from the screen in an operating tube. The photosensitive coating is then exposed to the point source of light, and the screen is developed by washing off the excess slurry that is not exposed.

Applying the process particularly to a cathode ray tube having an internal shield attached to the shadow mask assembly, which is mounted by the springs to three studs in the faceplate panel, the arm pivoted by the weight engages the shadow mask to tilt it so as to change the mask to faceplate spacing to equal the position that the moment caused by the weight of the shield causes the shadow mask assembly to be tilted to about the three studs, when the tube is mounted into a television receiver. Therefore, when the photosensitive material is exposed through the apertures of the shadow mask screen in the lighthouse, the phosphor dot triads are deposited on the screen in the proper position to be illuminated by the spots during tube operation.

DESCRIPTION OF THE DRAWING FIG. 1 is a side elevation view of a color cathode ray tube in accordance with this invention;

FIG. 2 is an expanded rear elevation view in cross-section taken along the lines 2-2 of FIG. 1;

FIG. 3 is a perspective view of a fragmentary portion of a shadow mask assembly and screen;

FIG. 4 is a schematic representation showing electronic beam landing on the triad phosphor dots of the cathode ray tube of FIG. 1;

FIG. 5 is a side elevation view in cross-section of a lighthouse for exposing the photosensitive coating on the front panel of a cathode ray tube;

FIG. 6 is a schematic representation illustrating the relation of the shadow mask assembly to the faceplate panel in accordance with the principles of this invention;

FIGS. 7 and 8 are diagrammatic representations illustrating the effect on the electron beam caused by shifting the shadow mask assembly relative to the faceplate panel; and

FIG. 9 is a schematic representation illustrating the results of the correction to beam landing error in accordance with the process of this invention.

DETAILED DESCRIPTION Referring to the drawings, FIGS. 1 and 2 show a color cathode ray tube 10, which has three electron guns 11 mounted in the tube neck 12, as is well-known in the art. A front panel 14 is attached to the funnel 16 of the tube to close the tube envelope and to provide a screen for the electron beams to illuminate. An internal shield 13 similar to the one described in copending application Ser. No. 570,888, filed Aug. 8, 1966 and now abandoned, and assigned to the assignee of this application, is connected to the shadow mask assembly 18. The shadow mask assembly 18 is connected in a spaced relation to the faceplate panel 14 by the three leaf springs 20a, b and c. The springs 20 are connected to metallic studs 22a, b and a respectively, which are embedded in the sides of the faceplate panel 14 to provide a three point suspension of the shadow mask in the known manner. Bimetallic strips 21a, b and c are welded to the mask and the springs are connected to the strips. The bimetallic strips are used for temperature compensation of the shadow mask during tube warmup in the known manner.

The shadow mask (FIG. 3) comprises a large number of apertures 24 etched into a thin metal sheet, and the screen 25 consists of a great number of triads of red, green and blue phosphor dots disposed on the inside of the front panel 14. Because of the location of the three electron guns on the order of an equilateral triangle in the tube neck 12, the beam from each gun approaches the shadow mask from a different angle so that each beam sees only its own set of phosphor dots in the triad. Each of the beams pass through a deflecting field as shown in FIG. 4, and the phosphor dots on the screen see a point in the field where each beam appears to originate. This source for each beam is called the color center or apparent source of the beam.

As shown in FIG. 3, the beam of each electron gun covers more than one aperture in the shadow mask and excites several phosphor dots associated with it simultaneously. In order to provide tolerances for inherent errors in the system, the shadow mask apertures have smaller diameters than the diameters of the color dots so that there is a guard ring about the color dots with the beam being concentrically aligned therewith. This guard ring guards against the electron beams writing off color even when there is some beam landing error present.

Even with a guard ring there are some errors introduced into the system which can cause the electron beams to write off color. The schematic representations in the drawings illustrate one of these errors and how it is cor rected in accordance with this invention.

One source of misregistration between the beam spot and the phosphor dot in a cathode ray tube using a three point suspension system as described is caused by the weight of the internal shield setting up a moment to cause the shadow mask to exert force on the springs 20. The shadow mask tilts about the three studs and shifts with respect to the screen. As indicated in FIG. 4, this moment of force causes the shadow mask to be tilted about the studs 22 such that the top of the shadow mask 18 is farther from the screen 25 than the bottom of the shadow mask. That is, the distance represented at a is greater than the distance represented at b. The electron beams represented by the line 27 emanate from the apparent source or color center 29 in the deflecting yoke field, pass through the shadow mask and strike the screen 26 in the upper left hand corner, as shown by the pattern at 30. It can be seen that the center of the beam spots 32 is to the left and above the center 34 of the phosphor dot triad, resulting in a misregistry of the beam spots 36 with respect to the phosphor dots 38. The scan of the electron beam from the color center 29 to the lower left corner of the screen is indicated by line 40. At this point in the pattern, as shown in the diagram at 42, the center 44 of the beam spots is moved to the right and above the center 46 of the phosphor dots 48, once again resulting in a misregistration of the beam spots with the phosphor dots. It should be noted that tilt of the shadow mask 18 about the studs 22 as shown in FIG. 4 is over-emphasized for illustrative purposes and in actuality the movement is somewhere on the order of 1 mil.

Any errors introduced in beam landing with the phosphor dots is magnified at the edge of the screen because of the large scan angle. It is for this reason that this is the area where substantial misalignment between the beam spots and the phosphor dots can be seen.

This invention prescribes the unique process and apparatus for correcting the beam landing error described. In general, the process for fabricating a screen for a color television tube includes the steps of depositing a photosensitive slurry which includes a phosphor (a luminescent material) suspended in polyvinal alcohol, which is sensitized with ammonium dichromate, or the like. The slurry is dried, and the shadow mask is inserted in a spaced relation to the front panel of the tube. The combination is then inserted in apparatus where the slurry is exposed to a light source which is positioned at the proper color center for the color involved. The light hardens the slurry behind the aperture in the areas illuminated by exposure to the light source. The mask is then removed and the screen is developed by washing with Water which removes the dried slurry in the areas not exposed. This process is repeated two more times. Each time a different color phosphor is added to the slurry until the blue, green and red phosphor dots have been deposited in a triad on the screen of the front panel. Applicant has conceived and developed a modification of the above method and the apparatus for accomplishing this modification, which provides correc tion to compensate for beam landing errors caused by the change in spacing between the shadow mask and the screen in an operating tube and the spacing between the shadow mask and screen when exposed in a lighthouse.

Referring to FIG. 5, there is shown a standard type lighthouse 60, which includes an open top cabinet 62 having an annular support base 64 on which the front panel 14 and the shadow mask assembly 18 can be mounted. The faceplate panel and shadow mask are positioned on the annular base 64 so that the spring 20a, at the top of the front panel, is located as shown. The end 66 of the shadow mask assembly, which is not connected to the faceplate panel, is also positioned as shown. About the support base 64 are a plurality of bosses 68 which function to position the faceplate panel 14 in a prescribed orientation on the lighthouse 60. At the base 70 of the cabinet there is located a housing 72, containing a lamp which provides a point source of light for exposing the screen 25 through the shadow masks 18. The housing 72 is mounted on a rotatable table 74, which permits the positioning of the light source at a plurality of positions. The spring loaded plunger assembly 76 provides means for locking the table 74 at the desired position.

In accordance with this invention, an arm 78 having a weight 80 on one end thereof is pivotally mounted to the frame 62 of the lighthouse 60. The weight is adjustably mounted to the arm 78 so that the moment of force available at point 82 of the arm may be varied. Also attached to the frame 62 in a spaced relation with respect to the arm 78 is an air cylinder 84, which is actuated from a compressed air source 86. An actuating plunger 86 is operated by the cylinder 84. In the position shown, the plunger is withdrawn into the cylinder, and the Weight 80 is free to pivot the arm 78 against the shadow mask assembly 18. With the plunger 86 extended by the cylinder 84 as shown in the dotted position, it engages the arm 78 and pivots it away from engagement with the shadow mask assembly 18.

In practicing my invention, after the photosensitive slurry has been deposited on the screen of the faceplate panel and the shadow mask inserted, the assembly is mounted to the lighthouse 60. Prior to exposing the screen 25 to the light source 72, the cylinder 84 is actuated and the weight, which in this example is on the order of 1 /2 pounds, moves the arm 78 ti engage the point 82 with the shadow mask. This applies a force to the shadow mask to tilt the same about the studs 22 such that the bottom of the shadow mask 66 (FIG. 6) is moved closer to the screen 25 of the front panel 14. The same rotating motion moves the top of the shadow mask 90 away from the screen 25. Therefore, the distance 0 that the shadow mask 18 is spaced from the screen 25 at 90 is greater than the distance d, which the shadow mask 18 is spaced from the screen 25 at 66. The beam 92 from the light source 72 is shown declined from the center line 91.

The effect of moving the shadow mask with respect to the screen can be seen by referring to FIG. 7 and viewing the beam 92 from the side, as if it were in vertical scan. Line f represents the bottom of the shadow mask at 66 with the shadow mask mounted to the lighthouse but without the force of arm 78 shifting it with respect to the faceplate panel. The shadow mask has an aperture 94 therein through which the beam 92 from the light source 72 passes on the way to the screen 25. The beam 92 passing through the aperture 94 strikes the screen and covers an area having a diameter substantially as shown at 96. The dotted line g represents the bottom of the shadow mask 18 after it has been moved closer to the screen 25 by the rotation of the mask about the studs 20 caused by the force of weight 80 applied through arm 78. The beam from the light source now travels a greater distance to reach the shadow mask g. The beam extension shown in dotted line passes through the aperture now labeled 94a in the shadow mask g and strikes the screen 25 with a beam diameter 96a. It is clear by referring to beam diameter 96 and beam diameter 96a that the eifect of moving the shadow mask 18 closer to the screen 25 is to cause the beam landing area to rise on the screen 25. Conversely, when the beam indicated at 93 is scanned at an angle above the centerline 91, moving the shadow mask 18 at the top closer to the screen will result in the beam landing area being lowered on the screen 25. In addition, reversing the procedure and moving the shadow mask away from the screen 18 at 66 will have just the opposite effect; i.e., if the mask is moved farther from the screen the beam will be shifted in a downward direction on the screen and, of course, if the beam is being scanned at an angle above the center line 91 and the mask is moved farther from the faceplate panel, the beam will be shifted in an upward direction on the screen.

Referring to FIG. 8 and viewing the beam 92a from above, as if it were during horizontal scan at the lower right corner of the screen 25a, a further effect of moving the shadow mask 7" closer to the screen, as at g can be seen. The shadow mask aperture 94b in the mask g is moved to the right (in the figure) with respect to the aperture in mask f. The beam 92a now striking the shadow mask will pass through the opening 94b and land on the screen 25a at the position represented by diameter 96b, which is to the left of the beam landing passing through the aperture 94b with the mask farther from the screen. Conversely, moving the mask farther from the screen will cause the beam landing to shift to the right. Furthermore, the effect is just reversed on the left hand side of the screen, i.e., moving the mask closer to the screen will cause the beam landing to be shifted to the right, and moving the mask away from the screen will cause the beam to be shifted to the left.

It is desirable in order to maintain maximum phosphor dot diameters and for ease of inspection that the dots be deposited on the screen ina tangential relationship with each other. In order to accomplish this within the precepts of this process, it is necessary that each of the three times that the screen is exposed in the lighthouse an equal force is applied to cause the shadow mask movement so that the triad of dots are all moved the same amount and in the same direction so that they can maintain this relationship. In effect, keeping the three dots tangent and moving them in the same direction results in moving the common center of the triad, such as shown at 34 in FIG. 4.

FIG. 9 illustrates how the correction is made to correct the error induced by the tilting of the shadow mask about the studs due to the weight of the internal shield working on the mask when the tube is mounted in a television receiver. When the front panel and shadow mask assembly are mounted on the lighthouse, the arm 78 is engaged with the shadow mask and the moment of force at 82 causes the mask to tilt so that the top part of the mask is moved farther away from the screen and the bottom closer thereto. Ideally the tilting of the mask is equal in amount and direction to that which occurs when the weight of the mask causes shifting of the mask in an operating tube. The result is frankly startling. FIG. 9a shows a typical triad 100 sampled in the lower left hand corner of the screen showing the error induced by the shift of the shadow mask about the studs caused by the internal shield, before the correction introduced in the lighthouse in accordance with this invention. It should be noted that the center 102 of the beam spots is above and to the right from the center 104 of the phosphor dots. By applying the force of weight 80 to tilt the mask about the studs 22, the bottom of the mask is moved closer to the screen so that the center of the exposed triad of phosphor dots is likewise shifted to the right and raised slightly as shown in FIG. 9b.

FIG. 9b illustrates the optimum condition which can be achieved by this process where movement of the shadow mask to shift the center of the deposited phosphor dot triad to the right and up has exactly compensated for the undesired tilting of the shadow mask about the studs such that there is little or no noticeable beam landing error and maximum guard band while maintaining tangent dots.

What has been described therefore is a unique process and apparatus for carrying out the process for depositing phosphor dot triads on the screen of a color CRT, for correcting beam landing errors. The dots are tangent to one another and can be easily inspected thereby increasing reliability and decreasing production time.

I claim:

1. A method of depositing a pattern of phosphor dots on the screen of the front panel of a color cathode ray tube to reduce errors between the landing of an electron beam on the screen projected from the origin of the beam source through apertures in a shadow mask assembly mounted in a predetermined spaced relation from the screen and the phosphor dots deposited thereon, the errors induced by the shadow mask assembly in an operating installation shifting about its mounting to alter the spaced relation between it and the screen, including the steps of, depositing a photosensitive coating on the front panel, positioning the front panel and shadow mask assembly in a predetermined spaced relation to a point source of light, moving the shadow mask assembly from the predetermined position relative to the screen to provide a spaced relation therebetween which is similar to that in an operating installation with the mask assembly being shifted about its mounting, exposing the photosensitive coating to the point source of light through the apertures in the shadow mask assembly subsequent to the same being moved to form dots on the screen that are substantially in alignment with the beam as it lands on the screen in an operating installation, and developing the exposed coating.

2. The method of claim 1 wherein each step is repeated three times so that a triad of phosphor dots is deposited on the screen, and the shadow mask assembly is moved relative to the screen an equal amount each time so that the dots in the triad are tangent to one another.

3. A method of depositing a pattern of phosphor dots on the screen of the front panel of a color cathode ray tube having a shadow mask assembly connected to the faceplate panel by a three point suspension system, and an internal shield connected to the shadow mask assembly, the method reducing beam landing errors caused by the shadow mask rotating about the suspension points to change the spacing between the screen and the shadow mask in an operating installation including the steps of,

depositing a photosensitive coating on the front panel, positioning the front panel and shadow mask assembly in a spaced relation to a point source of light, shifting the shadow mask assembly relative to the screen so that the spacing between the shadow mask and the screen is substantially the same as the spacing therebetween in an operating installation, exposing the photosensitive coating to the point source of light, and developing the exposed coating.

4. The method of claim 3 wherein the three point suspension ssytem includes bimetallic strips connected to the shadow mask assembly and coupling the assembly to the front panel.

5. The method of claim 3 wherein each step is repeated three times so that a triad of phosphor dots is deposited on the screen, and the shadow mask assembly is shifted the same amount each time with respect to the screen so that the dots in the triad are tangent to one another. I

6. A method of depositing a pattern of phosphor dots on the screen of the front panel of a color cathode ray tube having a shadow mask assembly connected to the faceplate panel with a three point suspension system including a plurality of bimetallic strips connected to the shadow mask assembly and coupling the same to the front panel, and an internal shield connected to the shadow mask assembly,.the method reducing beam landing errors caused by the shadow mask rotating about the suspension points and changing the spacing between the screen and the shadow mask in an operating installation, including the steps of, depositing a photosensitive coating on the front panel, positioning the front panel and shadow mask assembly on a lighthouse, engaging the shadow mask assembly with an arm weighted at one end which is pivotally mounted to the lighthouse and applying a force to the shadow mask assembly with the arm to move the shadow mask with respect to the faceplate panel thereby changing the spacing therebetween to be substantially the same as the spacing therebetween in an operating installation, exposing the photosensitive coating to the light source in the lighthouse, and developing the exposed coating.

References Cited UNITED STATES PATENTS 2,950,l93 8/1960 Payne 9636.1X 3,070,441 12/1962 Schwartz 9636.1 3,080,231 3/1963 Perry et a1 96--36.1 3,282,691 11/1966 Morrell et al 96-36.l

DAVID KLEIN, Primary Examiner US. 01. xx. 9s 1 

